The present invention relates to an internal-combustion engine having a crankcase with at least one cylinder to accommodate a piston, as well as to a method of producing such a crankcase.
Nowadays, crankcases for internal-combustion engines are predominantly produced of light-metal materials by die casting. In this case, Al—Si alloys are normally used. The ability to process such alloys by die casting is limited to hypoeutectic Al—Si alloys. By means of Al—Si alloys, crankshafts can be produced very cost-effectively and in large piece numbers by die casting.
A die-cast cylinder surface does not durably withstand the tribological stress in the piston/piston ring cylinder system. On the one hand, die-cast crankcases have a relatively high porosity. On the other hand, because of the relatively low strength of hypoeutectic Al—Si surfaces, their relatively high ductility and their insufficient resistance to wear, the tribological stressing capacity of hypoeutectic Al—Si surfaces makes these hypoeutectic Al—Si surfaces unsuitable for use as cylinder running surfaces. For achieving sufficient stability, one therefore frequently resorts to grey cast iron liners, which are inserted into the cylinders of light-metal crankcases.
As an alternative, light-metal crankcases are known whose cylinder running surfaces are coated with a suitable surface material. U.S. Pat. No. 5,908,670, WO 9749497, EP 568 315 B1, U.S. Pat. No. 5,626,674 and U.S. Pat. No. 5,380,564 describe corresponding coating processes, wherein first the cylinder running surface is roughened by means of a high-pressure fluid jet, and subsequently a coating in the form of melted-on metal or alloy droplets is applied, for example, by wire arc spraying, onto the roughened inner faces of the cylinders.
The relevant state of the art also includes a technical publication entitled “Thermal Spraying of Cylinder Bores with the Plasma Transferred Wire Arc Process” by K. Bobzin, F. Ernst, K. Richardt, T. Schlaefer, C. Verpoort, G. Flores, Surface and Coatings Technology, Volume 202, Issue 18, Jun. 15, 2008, Pages 4438-4443, as well as a publication entitled “Thermal Spraying of Cylinder Bores with the PTWA Internal Coating System” by K. Bobzin et al., Proceedings of the ASME Internal Combustion System Division, Fall 2007 Technical Conference, ICEF07, Oct. 14-17, 2007, Charleston, S.C., USA.
It is an object of the invention to provide an internal-combustion engine crankcase having at least one cylinder whose running surface is coated, in which case the running surface is to have a high tribological resistance.
This and other objects are achieved by an internal-combustion engine having a crankcase, which has at least one cylinder for accommodating a piston, the inner face of which cylinder is provided with a coating forming a running surface for the piston. The coating has a plurality of pores, the average size of the pores and/or the pore surface proportion varying over the length of the cylinder.
It may, for example, be useful to “adjust” the average pore size and the pore surface proportion such that the average pore size and the pore surface proportion decrease from the lower cylinder end in a direction toward the upper cylinder end. The “upper cylinder end” is the end on which the cylinder head is mounted. The lower cylinder end is the end facing away from the cylinder head.
As an alternative, it may be advantageous to adjust the size of the pores and the pore surface proportion such that they are largest in a center area of the cylinder and decrease in the directions toward both the upper and lower ends.
As an alternative, it may be provided that the pore surface proportion increases from the upper cylinder end in the direction toward the lower cylinder end and that the average pore size is essentially constant over the length of the cylinder.
As an alternative, it may be provided that the pore surface proportion is smallest in the center area of the cylinder and increases toward the two cylinder ends and that the average pore size is essentially constant over the length of the cylinder.
The process according to the invention for producing a crankcase for an internal-combustion engine having at least one cylinder particularly has the following steps:
(a) casting of the crankcase of a light-metal material, such as an aluminum silicon alloy, by die casting (especially hypoeutectic aluminum silicon alloys are considered for this purpose);
(b) subsequently, precision-turning the inner face of the at least one cylinder;
(c) then, roughening the precision-turned inner face; and
(d) finally, applying a coating to the roughened inner face, which coating forms a running surface for a piston to be inserted into the cylinder. According to the invention, the application of the coating takes place in such a manner that a coating with a plurality of pores is created, the average pore size and/or the pore surface proportion varying over the length of the cylinder.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.